DLibdavisking/dlib
DLib has C++ and Python interfaces for face detection and training general object detectors.

opencv/opencv
OpenCV has C++, C, Python, Java and MATLAB interfaces and supports Windows, Linux, Android and Mac OS.
DLib has C++ and Python interfaces for face detection and training general object detectors.
A machine learning API and server written in C++11. It makes state of the art machine learning easy to work with and integrate into existing applications.
A deep learning framework developed with cleanliness, readability, and speed in mind.
A real-time multi-person keypoint detection library for body, face, hands, and foot estimation
Lightweight, Portable, Flexible Distributed/Mobile Deep Learning with Dynamic, Mutation-aware Dataflow Dep Scheduler; for Python, R, Julia, Go, JavaScript and more.
The Computational Network Toolkit (CNTK) by Microsoft Research, is a unified deep-learning toolkit that describes neural networks as a series of computational steps via a directed graph.
An offline speech recognition toolkit with C++ support, designed for low-resource devices and multiple languages.
A C++ library for unsupervised text tokenization and detokenization, widely used in modern NLP models.